M. Johansson

Despite its importance for placental function, syncytiotrophoblast Na⁺-K⁺-ATPase has not been studied in detail. We purified syncytiotrophoblast microvillous (MVM) and basal (BM) membranes from full-term human placenta. Western blotting with isoform-specific antibodies demonstrated the presence of the α₁-subunit, but not the α₂- or α₃-subunits, in MVM and BM. Relative density per unit membrane protein in BM was 48 ± 1% (mean ± SE, n = 4, P < 0.02) of that in the MVM. The activity of Na⁺-K⁺-ATPase was lower in BM (1.4 ± 0.14 μmol · mg⁻¹· min⁻¹, n = 8, P < 0.02) than in MVM (3.9 ± 0.25 μmol · mg⁻¹· min⁻¹). Immunocytochemistry confirmed the distribution of Na⁺-K⁺-ATPase to MVM and BM. These findings suggest that the syncytiotrophoblast represents a type of transporting epithelium different from the classical epithelia found in the small intestine and kidney, where Na⁺-K⁺-ATPase is confined to the basolateral membrane only. This unique polarization of the Na⁺pump does not, however, preclude a net transcellular transport of Na⁺to the fetus.